Moment Arms and Exercise Progression (physics)

Progressive Overload

Exercise progression plays a vital role in all strength training programs. The concept of “progressive overload” hinges on this very concept of. As we all know, without some type of progression the athlete’s development will most likely stagnant and possibly begin to decline. This is why we increase weight in exercises. The weight adds overload in a progressive fashion. However, weight is not the only way we can manipulate our training to increase the demands of the body.

Exercise Progression

We can also progress the athlete by putting them in more demanding body positions. This can be done by manipulating the movement’s moment arms. A moment arm is the distance between the force acting on the body and the axis of rotation. This explains why holding a broom the “long way”, parallel to the ground is quite difficult, despite the broom weighing no more than a couple of pounds. The broom’s moment arm is greatly increased (the distance from the should to the broom’s downward force has increased) and thus puts a larger demand on the shoulder.

Moment Arms

Moment arms play a large role in athletic movements. Depending on joint angles, bone lengths, and body positions moment arms during movement will vary.

Without diving too deep into all of the biomechanics/physics calculations, we are going to look at how moment arms influence torque on the body. Torque is a rotary “twisting” force that acts on the body.

Torque = Moment * Force

Moment = Distance * Force

If force remains constant, we know that distance will be the contributor to a larger torque. Yes, changing force will change the moment, but for the sake of this example we are going to assume force is constant (no change in load) and only distance is going to change.

Back Extension

The exercise we are going to examine is the static hold back extension. This exercise is a good example, because it provides easy to see visualizes. We are going to use the athlete’s arm, to increase or decrease the moment arms of the movement.

The only variable force in this example is going to be the downward gravitational force of the arms of the athlete. We are going to assume the torso is going to be in the same spot the whole time.

Arms close to body

The first example is the athlete having their arms across their chest close to their body. We are going to use this as the standard when we compare the other modifications.

Arms partially extended overhead (no picture)

This example the athlete has their arms partially extended overhead. Here the moment arm is increased compared to the first example (picture above). The distance of the arms from the body increases. This means in the moment equation (above) the distance variable will increase, which in turn increases the torque and puts a greater demand on the torso to maintain body posture.

Arms fully extended

This example is the where the moment arms are at their greatest. Here the athlete has their arms fully extended overhead. The increase in moment arm (distance from the body) means that this position compared to the other two examples will have the greatest demands on the torso.

Using Moment Arms To Cause Perturbations

Actively changing moment arms during a movement can create perturbations. These perturbing forces increase the “dynamic” demand on the body, meaning the torso has to react to fluctuations in force. A way to create these perturbations in the back extension would to aggressively press and pull your hands your above your head and back towards your body (changing moment arms).

Progression

After reviewing the above examples, it should be easy to see how moment arms influence the demands of the movement. Knowing this, it should be clear that manipulation of these moment arms can be used to increase or decrease the difficulty of the movement. By simply adjusting moment arms, you can make a relatively easy exercise much more difficult. Understanding some of the basic principles of physics can help you guide your exercise progressions.

 

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